In modern seismic exploration, it has become common practice to employ a vibrator transducer to impart to the ground an oscillatory sweep-signal of predetermined characteristics. Generally the vibrator transducer includes a base plate in contact with the ground, a reaction mass having a weight of a ton or more, and a linear actuator designed to move the reaction mass reciprocatingly relative to the base plate in short vertical strokes. Working against the reaction mass, the base plate injects the desired vibrations into the ground.
For transporation from place to place, the vibrator is mounted on a vehicle in that way that it can be lowered by means of a lifting mechanism until the base plate is brought in contact with the ground, and thereafter lifted again to an elevated position for transport action.
Generally the vibrator is a two-masses-system which may be excited to oscillations, for instance by means of hydraulic pressure. One of these two masses is formed by the base plate together with the ground coupled thereto by contact. This mass is in connection with the reaction mass of the vibrator which forms the second mass of the system, by hydraulic means which are controllable by a combination of electric and hydraulic control means. These control means receive an electric signal from a registration station which is transformed into a seismic signal through mechanical oscillatory motion generated by the vibrator and emitted into the earth by the base plate.
Operations consist of a series of cycles carried out in a very short time each cycle going over a few seconds only. During one cycle a seismic signal will be emitted and received again for instance by geophones, the vibrator will be lifted, the carrier or vehicle moved to a new place, and the vibrator lowered into operational position at that new place.
Lifting devices of conventional design show several drawbacks which may lead to difficulties when seismic explorations are carried out.
Conventional lifting devices comprise two or four hydraulic jacks which are symmetrically mounted at both sides of the carrier vehicle. The lifting units are expressive and delicate devices and their strokes or lifting and lowering motions must be in precise synchronization for realizing the predetermined cycle. In spite of additional installations for ensuring the desired synchronization, damages cannot be avoided which are caused by tilting of the pistons in the hydraulic cylinder; this might be due to pressure variations, reaction forces etc, and cannot be reliably excluded in conventional system.
A further rather important drawback of conventional systems is that the delicate lifting units are mounted on the upside of the base plate and adjacent to its side edges. Mounting of the lifting jacks next to the vibrator is complicated also in this respect that access to the vibrator mass and associated parts will be difficult. This makes servicing a difficult and time consuming job.